1. Field
The present disclosure relates generally to fabricating fin field effect transistors (finFETs), and, more specifically, to fabricating finFETs with replacement sources and drains.
2. Description of Related Art
A finFET is a non-planar multi-gate transistor that operates on the same principles as traditional metal oxide semiconductor field effect transistors (MOSFETs). An island or fin of silicon is first patterned on to the wafer. After forming the fin, a gate stack is deposited and patterned so that the patterned gate material runs perpendicular to the fin. Where the patterned gate material overlaps the fin, a gate for the finFET is formed. The finFET will have one gate on both vertical sidewalls of the fin. Depending on the size of the top surface of the fin, the finFET may also have a gate on top of the fin.
For example, FIG. 1 depicts finFET 100 formed on a silicon-on-insulator (SOI) wafer. Fin structure 102 protrudes from oxide layer 108 and is raised above shallow trench isolation (STI) 106. Gate stack 104 contacts fin structure 102 on three sides forming multiple gates. A channel region is defined by the region under gate stack 104 in fin structure 102. Source and drain regions are adjacent the channel region at opposite ends in fin structure 102.
FIG. 2 depicts traditional MOSFET 200 formed on oxide layer 208 of an SOI wafer. In contrast to finFET 100 (FIG. 1), silicon region 202 is planar with STI 206, which causes gate stack 204 to contact only a single side of silicon region 202 and form only a single gate.
The multiple gates of finFETs may offer many improvements over traditional MOSFETs. For example, finFETs may be more robust against short-channel effects and may offer higher current drive while improving subthreshold swing.
However, the non-planar nature of finFETs may lead to several difficulties during fabrication. For example, conventional implanters require line of sight to implant ions into a surface. To dope vertical sidewalls of the source or drain of a finFET, the wafer must be tilt-angle implanted. To overcome shadowing effects from nearby finFETs, the implant may need to be performed at multiple angles or orientations, which may increase implanter tool complexity, implanting cost, and processing time. Alternatively, design rules may be implemented that space non-planar structures far enough apart to minimize shadowing effects. However, using design rules to increase structure spacing will result in less dense circuits.
One alternative to tilt-angle implants is plasma doping. However, plasma doping may have its own set of disadvantages, such as doping control and limits on doping concentrations.